30 Papers from the Marine Biological Laboratory at Tortugas. 



I was hidden from view. Often while observing ocypodas in the trap I 

 have found that they did not move when a stick was waved in front of 

 them, but as soon as it hit the side of the trap they would react either by a 

 quick jump or a movement of the eye-stalks. However, there was con- 

 siderable difference in the behavior of individuals, some being quite sensitive 

 and others not so much so. 



Ocypoda also reacts to vibrations of the water in aquaria, but it is not 

 nearly so sensitive to these stimuli as to those caused by jarring the sand. 



LOCOMOTION. 



The locomotion of the Decapoda has been investigated by List (1897) 

 and Bethe (1897), but both of these workers have confined their studies 

 to those forms which live most of the time in water and which are not 

 adapted to very active locomotion on land. The Ocypodje, however, spend 

 a large part of their lives on the beach sands and can travel for long dis- 

 tances at a considerable rate of speed. In fact, their movements are so 

 rapid that it is often impossible to determine the order in which the various 

 ambulatory appendages are used. 



Bethe (1897) has observed that Carcinus tiuenas ordinarily travels side- 

 ways, but that to a very limited extent it can move forward and backward. 

 According to this investigator the locomotion sideways is directly sideways, 

 and not oblique as List (1897) has described for decapods in general. 



As in the case of Carcinus, ordinarily Ocypoda runs sideways and in a 

 direction at right angles to the sagittal plane of the body. It seems to be 

 much better adapted to locomotion in other directions, however, than any 

 of the Brachyura. Unlike Carcinus, it will move obliquely sideways with 

 considerable speed and will travel forward for long distances at a good 

 rate. As mentioned in Experiment D, Ocypoda can move backward for 

 quite a distance, but it does so slowly, and when a quick movement is re- 

 quired in that direction it usually jumps. On the whole it seems to be better 

 adapted to life on land, so far as locomotion is concerned, than any other 

 decapod. 



Ocypoda usually runs toward an object with a considerable degree of 

 accuracy. It often returns to its burrow almost directly in a straight line 

 when disturbed by the approach of man, and I have often seen them ac- 

 complish this from a distance of 15 or 20 feet. This same accuracy is 

 shown when they approach objects which have been thrown on the sand, 

 although it is true that sometimes they fail to go more than two-thirds of 

 the distance. So far as I have observed, individuals very seldom run beyond 

 the mark. 



Bethe (1897) on page 510 says: "Mann sicht daraus klar dass von 

 einem ' Sehen ' in unserem Sinne, von einer Perception der Lage und Ent- 

 fernung des Gegenstandes, nicht die Rede sein kann." Bethe's statement 



